fix(doxygen): doxygen uses the correct README.md to generate main page

What does this MR do?

Update sight Doxyfile to fix the path of the main README.md. Now the doxygen is generated using the main README file as the main page.

How to test it?

• Generate the doxygen and see that the main page is correct:
  • Set BUILD_DOCUMENTATION=ON in your cmake options. Then ninja doc.
• Or use the doxygen generated by the CI: doxygen-390-doxygen-doxygen-uses-the-wrong-readme-md-to-generate-main-pp.zip

Associated Issues/Merge Requests

• Issues
  • Fixes #390 (closed)